1. Field of the Invention
The present invention relates to a lighting network, and more particularly to a lighting network in which the principal light source is a gaseous discharge light source for which a standby light source is needed during the low light output warm-up period or when it has gone out, and requires restarting. Such lighting networks require control means for sensing the condition of the principal light source, and for turning on the standy light source when the principal light source is not up to brightness.
2. Description of the Prior Art
It has been recognized that a gaseous discharge light source may need to be supplemented with a standby light source in some applications. It has also been recognized that the voltage across the gaseous discharge lamp in the conventional power supply can be an indirect indication of the light output of the gaseous discharge device. In particular, when the light source has gone out the voltage at the light source terminals will rise, since power supplies for gaseous discharge light sources to some degree must be current sources. Likewise, it is known, that once the arc in a gaseous discharge light source has ignited, which may require a voltage ten times the operating voltage, the voltage will immediately fall to a low value typically about one-third the normal operating voltage. Under this starting condition, current limiting in the power supply is mandatory to preserve the electrodes from self destruction. During the period of low arc voltage, the light output is low. As the starting period continues, the arc, which is first supported by the ionized argon or other natural gases, is supplemented by mercury which is initially a liquid. The mercury must next become vaporized and then ionized to enter into the light generation process. As the internal temperature continues to rise in the light source, the gas pressure builds up. With the growth in gas pressure, the voltage of the arc rises threefold to the normal voltage range. At the same time, the light source reaches its normal brightness. Should the light source go out, restarting is difficult, and normally requires time for the light source to cool to some degree. With cooling, the internal pressures fall, and re-ignition with reasonable voltages again becomes possible. A lamp which ignites at 1000 volts when cool may require 20,000 volts for ignition at high temperatures. Interruptions in line voltage on the order of a few milliseconds will invariably extinguish the light source, and then some delay will be required for cooling and recycling before full brightness can be restored.
In recognition of the relationship between the terminal voltage of a gaseous discharge light source and its light output, it has been proposed to connect a voltage breakdown device, such as a diac, in series with a standby incandescent light source in parallel with the gaseous discharge light source. When the gaseous discharge light source is lighted, the voltage across the terminals is insufficient to break down the diac, and the incandescent light source remains unlit. Should the gaseous discharge light source go out, the lamp terminal voltage will climb to a voltage adequate to break down the diac and turn on the incandescent light source. A diac shunt circuit has the disadvantage that it constitutes a load to any ignition voltage in excess of the disc breakdown potential, and poses problems for both the initial cold start and the hot restart, where the ignition potentials are even higher. Breakdown devices connected into the gaseous discharge lamp circuits have been proposed for sensing both low arc voltage and the high voltage resulting from extinction of the arc. Answers to the starting problem posed by a shunt connected breakdown device, have been to stack two to achieve higher breakdown potentials or to place a diode in series with the diac and the incandescent light poled to allow the standby circuit to load only the half cycles of the ac power source not used for developing the high ignition voltage.